Re: SRP Q's: 1) When is asynchronous I/O complete, 2) Is sequential I/O coalesced, and 3) why is iSCSI faster than SRP in some instances

[Vladislav Bolkhovitin <vst-d+Crzxg7Rs0@public.gmane.org>]

Chris Worley, on 01/09/2010 01:39 AM wrote:
>>> I thought if the device was opened with the O_DIRECT flag, then the
>>> scheduler should have nothing to coalesce.
>> Depends on how many I/Os your application has in flight at once,
>> assuming it is using AIO or threads. If you have more requests in flight
>> than can be queued, the block layer will coalesce if possible.
> 
> I do use AIO, always 64 threads, each w/ 64 outstanding I/O's.  Local
> or iSER initiator based, I never see any coalescing.  Only w/ SRP.

SRP initiator seems to be not too well optimized for the best 
performance. ISER initiator is noticeably better in this area.

> There is the scst_vdisk "Direct I/O" option that's been commented out
> of the code, as it's not supposed to work... maybe direct I/O doesn't
> work... but that would be the target side.

O_DIRECT for vdisk is supposed to work. It's a matter of a small patch 
for the kernel, see http://scst.sourceforge.net/contributing.html#O_DIRECT.

Meanwhile, you can use fileio_tgt handler, with which O_DIRECT works well.

Vlad
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